A semiconductor device that has a multilayer wiring structure is manufactured by repeatedly subjecting the surface of a silicon substrate or another such semiconductor wafer to film forming and pattern etching. For example, a floating gate layer, a control gate layer, and a control gate electrode layer are layered on the silicon substrate from the bottom layer up in a device known as a flash memory, and this layered film is patterned, thereby constructing a gate structure.
For the layered film, polysilicon, for example, is used for the floating gate electrode layer, and tungsten (hereinafter W), for example, or another such metal material and compounds thereof are used for the control gate electrode layer. These films are typically formed by CVD (Chemical Vapor Deposition), sputtering, or another such method. The film is patterned by using dry etching and physically shaving the surface of the layered film.
However, as this gate structure is reduced in thickness and size, the leakage currents between the layers become impossible to ignore. It has also become impossible to ignore leakage currents from side walls that accompany the miniaturization of the components. In the gate structure of a flash memory, the side wall portion is the most likely to have leakage currents, and the edge portions in particular are likely to leak due to electric field concentration. If the side walls are oxidized and the leakage currents can be reduced, the properties of the components are improved, but conversely, when the control gate electrode that uses metal elements is oxidized, the resistivity of the gate electrode portion increases, and needle-shaped crystals known as whiskers penetrate the adjacent film and grow, damaging the apparatus and leading to an unavoidable loss in component properties and yield rate. Also, the exposed metal portion simultaneously sublimates in the atmosphere during the substrate processing and adheres to the processing container or the substrate, causing unacceptable metal contamination to occur.
These problems would be resolved if the W could be replaced with an oxidation-resistant material, but a suitable material has not yet been found. Therefore, another way to resolve these problems is to find a method for oxidizing only the side walls of the polysilicon constituting the floating gate, without oxidizing the W used as the control gate electrode. Specifically, there is a need for a selective oxidation technique that would be able to selectively oxidize the silicon alone.
In view of this, a method that has been considered in conventional practice as a selective oxidation process is a method for selectively oxidizing only the silicon by using plasma that is a mixture of hydrogen gas and oxygen gas instead of moisture. An example of this conventional selective oxidation method will be described using FIG. 7. After a substrate has been conveyed into a processing chamber, the temperature of the substrate is raised to a specific processing temperature (substrate temperature increase), and a mixed gas (H2+O2 gas) containing hydrogen gas (H2 gas) and oxygen gas (O2 gas) is fed into the processing chamber while the pressure in the processing chamber is adjusted to a specific pressure (pressure adjustment). When the pressure in the processing chamber has stabilized, plasma discharge is initiated (discharge initiation), and the discharge is continued. This discharge creates H2+O2 mixed plasma, and this mixed plasma selectively oxidizes the side walls of the gate structure formed on the substrate surface (substrate processing). The supply of mixed gas and the discharge are both stopped (discharge stopping), and the processing chamber is brought to the same pressure (approximately 100 Pa) as a vacuum transportation chamber (substrate conveying-out preparation) in order to convey the substrate from the processing chamber. Thus, an attempt is made to selectively oxidize only the silicon by using mixed plasma containing H2 gas and O2 gas.
As a relevant technique, in the step of creating the silicon oxide film, Ar is first introduced in order to prevent initial increased oxidation because Ar has the property of being easily discharged, and then oxygen is introduced and the oxygen radicals are activated (for example, see Patent Document 1). Another technique is to clean with hydrogen in order to remove the naturally oxidized film (for example, see Patent Document 2). Furthermore, another technique is to perform selective oxidation by reducing the tungsten (W) (for example, see Patent Document 3).
Patent Document 1: JP-A 2003-163213
Patent Document 2: JP-A 2000-150479
Patent Document 3: JP-A 8-102534